The increasing variety of laser applications in recent years has led to an increased demand for laser sources having properties suited to individual requirements. Among the properties which characterize these laser sources are the monochromatic nature of the light produced, its directionality, power and coherence. Difficulties arise in design of laser light sources because achievement of increased levels of some of these properties often presents inconsistent design constraints. An illustration of this problem can be seen in relation to the design of a laser source for increased levels of power and directionality. A measure of the directionality of a laser light beam is its divergence-angle.
In general terms, a laser system includes an active medium which is located within an optical resonator for producing an oscillator beam. In a stable laser resonator, a widely used method of achieving a low divergence, corresponding to a highly directional beam, is to introduce an aperture to narrow the oscillator beam. Such a resonator is described as having a small Fresnel number. This in turn means that the value of D.sup.2 /L is small, where D is the diameter of the oscillator beam propagating inside the resonator and L is the resonator length.
For example, the lowest divergence, corresponding to the diffraction-limit, is achieved when D.sup.2 /L is of the order of magnitude of the wavelength of the laser. For a wavelength of 1 micron and a 1 m long resonator, the diameter of the intracavity aperture which determines the width of the oscillator beam is therefore limited to about 1 mm. However, with this small aperture, a narrow oscillator beam is produced during lasing action which utilizes only a small portion of the volume of the active medium. As a result of the limited exploitation of the active medium volume, the output power is substantially reduced and the laser is inherently inefficient. It would therefore be desirable to have a laser which achieves a high output power by exploiting more of the active medium volume while still maintaining a low divergence and high efficiency.
One solution that has been proposed for increased efficiency is to reduce the volume of the active medium through reduction of its diameter. This has the effect of increasing the proportion of the active medium volume which is utilized by the oscillator beam. This solution is used in some laser systems (e.g. He-Ne lasers and some CO.sub.2 lasers). In solid-state lasers, however, the use of a thin rod (e.g. 1 or 2 mm in diameter) to reduce the active medium volume is impractical because of mechanical strength considerations and because of poor coupling with the flashlamp used as the optical pumping source.
A better solution for utilization of the active medium is provided by the use of an unstable resonator. This method is described for example by Herbst et al in Optics Communications, volume 21, page 5 (1977). With this design, a very low divergence is obtained at a high output power and the whole volume of the active medium is utilized. An important drawback of this design, however, is the very high oscillation threshold which results in a low efficiency of the laser.
Another solution to the active medium utilization problem is based on the so-called "telescopic resonator" as described for example by Hanna et al in Optics Communications, volume 37, page 359 (1981). In this design, a beam expander is inserted into a stable resonator. On one side of the beam expander where the active medium is located, the oscillator beam is wide thereby utilizing the whole volume of the active medium. On the other side of the beam expander, the oscillator beam is narrow thereby maintaining a small effective Fresnel number of the resonator. This approach provides a high power, low divergence output beam at a good efficiency. However, an inherent disadvantage of the design is the very high power density in the narrow oscillator beam which is liable to cause damage to optical components on that side of the beam expander.
Still another prior art proposal for a solution to the active medium utilization problem is found in the disclosure in U.S. Pat. No. 3,852,684 (Roess et at). This patent discloses a laser device for generating a laser beam having a fundamental mode, which device uses a portion of a laser active material as the oscillator and the remaining portion as an amplifier so that the energy stored in the laser material is fully exploited. The laser device comprises a laser active material, a pair of small mirrors designed to form an optical resonator utilizing a portion of the laser active material with the remaining portion of the material being an amplifier portion. One of the pair of mirrors is partially transparent to the light so as to enable decoupling of the beam from the resonator. The device furthermore comprises a matching optical system including means for directing a laser beam decoupled from the optical resonator portion through the amplifier portion of the laser active material for amplification.
Another prior proposal, similar to that of Roess, is to be found in U.S. Pat. No. 4,276,519 (Marteau). Here, as with Roess, amplification is achieved and in consequence laser efficiency is improved. However in both cases power losses occur as a result of the fact that the expanded output beam, when returned to the active medium for amplification, is reflected along its output path so that at least a portion of it has to pass through the oscillator output mirror without 100% transmission. As a consequence, the high reflection losses in this output mirror substantially reduce the intensity of the expanded beam, thereby reducing the efficiency of the laser device. Whilst with such arrangements, by virtue of the fact that more of the volume of the active medium is used amplification is achieved, these designs suffer however, as indicated above, from losses inherent in the fact that return of the beam for amplification takes place through the partially transparent mirror.